A portable apparatus, such as a mobile phone, is provided with light emitting diodes (hereinafter described as LEDs) as a plurality of light emitting elements of different light emission colors for emitting light as backlight of an image display comprised of, for example, a liquid crystal device (LCD) or displaying an incoming call etc.
Current portable apparatuses are generally provided with red (R) LEDs, green (G) LEDs, blue (B) LEDs, and white LEDs.
These various types of LEDs have different forward voltages (Vf). For example, the forward voltage Vfr of the red LEDs is set at approximately 2.0V, the forward voltages Vfg and Vfb of the green and blue LEDs are set at approximately 3V, and the forward voltage Vfw of the white LEDs is set at approximately 3.5V.
Portable apparatuses mounting various types of LEDs having different forward voltages in this way have a LED drive apparatus for driving these LEDs.
The output voltage in this LED drive apparatus is set by selecting a value satisfying the forward voltage of the maximum value in order to handle the various types of LEDs having different forward voltages. For example, when red LEDs having the forward voltage Vfr of 2.0V and white LEDs having the forward voltage Vfw of 3.5V are driven by the same power supply, the output voltage of the LED drive apparatus is generally fixed to 4.5V to 5.0V by considering the variation of the voltage required for the constant current source and the forward voltage Vfw of the white LEDs.
When using a LED drive apparatus having the output voltage matched with the LEDs having the highest forward voltage, however, for example, red LEDs having a low forward voltage will be driven by a voltage higher than the required drive voltage by 2.0V. As a result, there is an accompanying very large power loss.
Further, as explained above, the apparatus is designed by including an operating margin considering the variation of the LEDs having the high forward voltage. This operating margin becomes one of the factors of power loss.
This problem of power loss lowers the light emitting efficiency of the LEDs remarkably. A portable apparatus is driven by batteries due to its portability, so this power loss will shorten the actual usage time of the portable apparatus.
For this reason, in conventional LED drive circuits, studies are being made for the purpose of raising the efficiency of the charge pump or DC-DC converter serving as the power supply. However, the efficiency of these circuits has already exceeded 90%. Therefore it becomes difficult to extend the actual usage time even if the efficiency is raised more than this.
On the other hand, there is a method of connecting a few LEDs in series and driving them by a boosted power supply as a means for solving the above problem.
By using this method, the output of the LED drive circuit is controlled to the voltage of the required lowest limit, so a high efficiency (high light emitting efficiency) can be expected.
However, this method involves the following problems.
First, since the output voltage becomes high, a high voltage resistance process becomes necessary.
Second, for an output within the voltage resistance, driving three to four LEDs is the limit.
Third, the LEDs are connected in series, so independent control of the LEDs is difficult.
The third problem is especially large. The function of “many LEDs emitting light in various ways” expected from portable apparatuses in recent years is not satisfied.